Ask Dave: Why does RV’s water heater work on LP but not on 120 volts?

Dear Dave,
Regarding my RV’s water heater, the gas part works good after I replaced the disc thermostats but I cannot get the electric to work right. It’s an Atwood 10-gallon unit. I have replaced the solenoid switch and the heating element, but it’s still not working. —Brad, 2006 Monaco Monarch

Dear Brad,
The very first thing you need to check is whether there is 120-volt power coming to the Atwood water heater. Power is supplied from the distribution center, where you will have a dedicated circuit breaker for the water heater. Verify there is 120-volt power at the circuit and going to the water heater.

Most RV manufacturers hardwire the Romex electrical cord to the water heater. However, I have come across some that have an actual outlet near the compartment and an actual plug-in cord coming off the water heater. In some cases it was simply that the cord was not plugged in.

Here is the back of a newer model we have in the shop, but it should be very similar. The heating element screws into the larger hole in the center. This model came from Winnebago, so it has a heat exchanger. Those are the two ports on the lower sides.

Here is the wiring diagram from the Atwood Service Manual:

Notice the 115V Hot (black) goes through a relay connected and controlled by the thermal cut-off, then to the Electric Heating Element. Then the neutral (white) is wired directly. The ground (green) comes in to the heating element, as well.

You should be able to verify 120-volt power to the back side of this heating element with the unit plugged in to an electrical source and the switch is on. If it works on the LP mode, then it is not the ECO switch, thermostat, or thermal cut-off. Also it is most likely not the module board, as the 120-v power does not go through it and is only affected by the relay which shuts off only by the ECO and T-stat. That would also shut off the LP operation.

If all the 120-volt troubleshooting shows power to the heating element, it is possible you have a defective new element. You should be able to verify power to the element and then do an Ohm test. But I doubt you need that if there is power.

This is a photo of a unit in a 2003 Winnebago Brave. It should be similar to what you are looking at. Not sure what “solenoid switch” you are referring to. However, it seems you did replace the ECO and T-stat, as they are the disc-shaped ones mentioned.

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Water heater works on 120-volt but not on LP. What to check?

Dear Dave,
I have a Suburban water heater with dual 120-volt/LP. It works fine on electric. However, when I try to use the LP, the pilot light lights and stays on. But when I try to turn the valve to “On” for the burner to light, the pilot light goes out. What should I check out? Thanks. —Virgil

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Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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